A conventional high power wideband amplifier using low output impedance emitter follower stages is shown in FIG. 1. Some commercially available rail-to-rail devices similar to this design are based on common emitter output stages as shown in FIG. 2. Conventional common emitter follower architectures suffer from high frequency-runaway caused by different signal delays through the output devices and by internal self rectification. Moreover, in devices with low output impedance thermal runaway can occur.
FIG. 3 shows a class B current conveyor based amplifier that can achieve high bandwidth. The amplifier shown in this figure preamplifies the input signal using an opamp IC3 and inputs the signal to the node connecting npn transistor Q12 and pnp transistor Q13. Each of transistors Q12 and Q13 is diode connected, so that a positive current excursion of the input signal causes a current to flow through Q12 resistor R20 and a negative current excursion of the input signal causes a current to flow through Q13 and R21. The thus created currents are mirrored by the current mirrors formed by transistors Q12 and Q14 and transistors Q13 and Q15 respectively to flow from the positive supply rail through the transistors Q16 and Q14 and resistor R22 to ground, if the voltage excursion of the input signal is a positive one, or through resistor R22 and transistors Q15 and Q17 if the voltage excursion of the input signal is a negative one. These currents are again mirrored by the current mirrors formed by transistors Q16 and Q18 and transistors Q17 and Q19 respectively to provide the output current IOUT. Both the class B current conveyor architecture shown in FIG. 3 and the class B second generation current conveyor architecture shown in FIG. 4 are bandwidth limited by the switching times of the output devices.